1. Field of the Invention
The present invention relates to a technique of optical exposure, particularly to a technique of optical exposure using ultraviolet light for forming a fine pattern.
2. Description of the Related Art
In semiconductor integrated circuit devices, the enhancement of integration and fine processing of the devices is under way. The processable line width is decreasing from submicron order to half micron order at present, and it is therefore desired to develop a technique of fine processing in half micron or smaller order.
For coping with the above requirement, the light source for photolithography is being shifted from a mercury lamp, g-line (436 nm) and i-line (365 nm), to KrF (248 nm) of an excimer laser and further to ArF (193 nm) thereof.
In the photolithography using g-line and i-line from a mercury lamp, it is general. practice to use a novolak resin material for a photoresist. Since, however, a novolak resin shows high absorption in a deep ultraviolet region, e.g., at 248 nm of KrF, it is no longer suitable for use as a resist material.
FIGS. 2A and 2B schematically show states in which a positive resist of a novolak resin is exposed to light from a KrF excimer laser.
In FIG. 2A, an etchable wiring layer 52 is formed on a semiconductor substrate 51, and a resist layer 53 of a novolak resin is formed on the wiring layer 52. A light-shielding mask 54 is placed above the resist layer 53, and the resist layer is exposed to deep ultraviolet light 55 at 248 rin from a KrF excimer laser. The deep ultraviolet light 55 comes into the resist layer 53 through an opening of the light-shielding mask 54.
Meanwhile, the resist layer 53 shows strong absorption to deep ultraviolet light at 248 nm. As deep ultraviolet light moves within the resist layer 53, its light amount greatly decreases as shown on the right in FIG. 2A. It is therefore difficult to expose the resist layer 53 to sufficient light all through the depth thereof.
When the resist layer 53 is developed after the above exposure, a resist pattern as shown in FIG. 2B is liable to be formed. That is, an opening 56a having intended dimensions is formed, while the opening becomes narrower in a tapered shape toward the interior of the resist layer, and no "through" pattern is formed, or an opening 56b in the bottom portion has only smaller dimensions than the opening 56a in the upper portion. When the so-formed resist pattern is used, no intended pattern is obtained, or the pattern accuracy greatly decreases.
As a substitute for the novolak resist, therefore, the development of a chemically amplifiable resist containing a base polymer having high transparency in a deep ultraviolet light region has been under way. The chemically amplifiable resist refers to a resist containing a base polymer and a photo acid generator. When the resist is exposed to ultraviolet light, the photo acid generator in the resist generates acid. When the resist is baked after the exposure, the generated acid works as a catalyst to deblock of polymer pendant groups, and the like.
The chemically amplifiable resist can contain a base polymer having sufficiently high transparency at the KrF wavelength, and can overcome the problem caused by extremely high optical absorption of a resist as shown in FIG. 2.
When, however, the resist has too high transparency, there is another problem caused by reflection from an underlayer. For example, when a processable layer having a high reflectance such as a layer of aluminum (Al) or tungsten suicide formed on an uneven surface is subjected to patterning, the reflection on the processable layer surface decreases the pattern accuracy.
FIGS. 3A and 3B show schematically show a problem caused by optical reflection on the processable layer surface. In FIG. 3A, a structure of an oxide layer 58 is selectively formed on the surface of a semiconductor substrate 57, and an uneven surface is formed thereon. A wiring layer 52 having a high reflectance is formed on the uneven surface, and a resist layer 59 is formed for patterning the above wiring layer 52.
A light-shielding mask 54 is placed above the resist layer 59, and deep ultraviolet light 55 is directed from above the light-shielding mask 54. The deep ultraviolet light 5 comes into the resist layer 59 through an opening of the light-shielding mask 54.
The resist layer 59 shows low optical absorption to the incoming ultraviolet light, and the deep ultraviolet light 55 comes through the resist layer 59 and arrives at the undersurface of the resist layer 59. The processable layer 52 having a high optical reflectance is disposed on the undersurface of the resist Layer 59. As a result, the deep ultraviolet light 55 is reflected on the processable layer 52 surface. When the substrate has an uneven surface, reflected light does not return through an optical path of incoming light, but light is scattered (reflected) out of the optical. path.
When convex portions are present on two sides as shown in FIG. 3A, deep ultraviolet Light incident on a slope is scattered toward a central portion which is to be shielded against light. As a result, the resist region which should be shielded against light is exposed to the light to decrease the pattern accuracy. A like phenomenon also takes place when the processable layer has an irregular reflection surface.
FIG. 3B shows the form of a resist pattern formed on an uneven surface as shown in FIG. 3A after development when a positive resist layer has too high transparency.
In that region of the resist layer 59 which is between the convex portions of the oxide layer 58 on the substrate 57 surface, the width of the resist pattern 59a is decreased in the above region slnce the above region is exposed to tight reflected from the slopes.
When the above-obtained resist pattern 59a is used for patterning a wiring layer (processable layer) 52 located on the undersurface of the resist pattern 59a, the width of the patterned wiring layer is so decreased in the region located between the convex portions of the oxide layer 58 as the width of the resist pattern 59a has been decreased in the corresponding region. When the reflection is intense as described above, no intended pattern is obtained.
The optical transparency of the chemically ampilfiable resist to light can be controlled by adjusting the amount of the photo acid generator. However, general photo acid generator show nearly constant absorption coefficients. When the amount of the photo acid generator is small, the effect of decreasing the reflection is insufficient. When the amount of the photo acid generator is too large, the resist shows too intense absorption, and the pattern profile is formed in a tapered pattern.
Further, the acid generated by the photo acid generator works as a catalyst in the deblocking of protective groups in the base polymer of the resist, while the resist shows low sensitivity when the amount of the acid is insufficient.
Further, there is known a phenomenon in which the upper portion of a resist pattern forms eaves when the chemically amplifiable resist, a positive type in particular, is used. FIGS. 4A and 4B show the phenomenon in which the upper portion of a resist pattern forms eaves.
As shown in FIG. 4A, a chemically amplifiable resist 62 is applied onto a processable layer 61, and the resist is exposed, for example, in a stripe (line and space) pattern.
When the chemically amplifiable resist layer 62 is developed after the exposure, the upper portion of a developed resist pattern 62a has eaves 63 having a form extending from the resist pattern as shown in FIG. 4B.
The above phenomenon is considered to take place for the following reason. When the chemically amplifiable resist layer 62 is exposed to deep ultraviolet light, the photo acid generator in the chemically amplifiable resist layer 62 generates an acid, and on the resist layer surface, contaminations such as an amine from air react with H+ generated from the photo acid generator and deactivate (neutralize) the acid to cause an insolubilization reaction. Or, the acid generated when the exposed resist is baked volatilizes so that the acid concentration in the vicinity of the resist surface decreases. When the amount of the photo acid generator is increased for preventing the formation of the eaves, the optical absorption is intensified to increase the tendeticy to form a tapered pattern.
As explained above, the transparency of the chemically amplifiable resist to incident deep ultraviolet light could be controlled by adjusting the amount of the photo acid generator. However, it is difficult to optimize the transparency of the chemically amplifiable resist on the basis of the chemically amplifiable resist amount alone.